Challenge Description
As the challenge name and hint suggest, the flag is probably encrypted using a caesar cipher. We first download the file using wget to see how the encrypted flag looks like to understand how we should obtain the flag.
Running cat encrypted.txt displays a string xqkwKBN{z0bib1wv_l3kzgxb3l_555957n3}. Observing this string allows us to conclude that we will need to use caesar cipher to decrypt it, due to the presence of the familiar curly braces {}.
Further analysis (or trial and error) tells us that the caesar cipher input value should be 18. From here, we can use CyberChef to decrypt the message.
Flag
picoCTF{r0tat1on_d3crypt3d_555957f3}